The response of pixels in linear image detectors such as focal plane arrays typically may vary in a random fashion by .+-.15%. This severely limits the fidelity of the recorded image. To improve the fidelity of an image produced by such sensors, it is common to calibrate the image sensor by illuminating the image sensor with a controlled source of uniform illumination and record the responses of the individual pixels in the sensor. For maximal fidelity, each pixel must respond identically to equal luminance levels. It is the function of light calibrators to ensure an equalization of pixel response to equal input (amplitude and spectral) The calibrator inputs known amounts of radiation energy into each pixel or detector element and its response to such input recorded. This is done for a range of calibrator controlled input radiation levels from zero to some maximal radiation level. The pixel response measured and recorded in terms of offset (bias) and gain (slope). These results are stored in a memory and subsequently are used to adjust pixel response in absolute and relative terms.
Current calibrators commonly use incandescent radiation (light) sources as the active element of the calibrator. Such sources have the following disadvantages:
1. The spectral characteristics of incandescent light sources are rich in the red part of the light spectra and thus are inherently poor simulators of sunlit objects which are common in the photographic process. PA1 2. Light output from an incandescent light source is specular and generally exhibits illumination hot spots, due for example to filament irregularity and shadowing, which result in a non-uniform illumination. PA1 3. The spectral output from incandescent lamps changes as a function of input current supplied to the filament and hence exhibit strong variations in the characteristics of the output light spectra with changes in light output level. Furthermore, the spectral output of incandescent lamps changes over time at a given output level due to evaporation and deposition of the filament material onto the lamp envelope. PA1 4. Incandescent light sources are compact and thereby require large separation from the image sensor or complex optical projection systems that spread the light to insure coverage. PA1 1. A wideband spectra equivalent to that supplied by natural daylight due to the ability to provide a phosphor mixture tailored to the spectral requirements of the sensor. PA1 2. The fluorescent light source is capable of providing a long uniform, diffuse light source such as is required for coverage of long linear image sensors. PA1 3. Stable light level control (relatively invariant spectral distribution with change in illuminance level). Electronic light level control is thus enabled. PA1 4. Superior life and reliability characteristics. PA1 5. The light output is essentially Lambertian and ripple free, being generated from phosphor excitation and hence inherently diffuse in character.